Electronic transport in hybrid heterostructures and universal control of spin-orbit interaction in quantum wells

In recent years, the control of charge and spin in semiconductors has experienced outstanding progress. The extended spin coherence times established in high quality materials together with the advance in elaborate fabrication methods enable coherent spin control, ultimately leading to new spintronic devices. In this thesis, we investigate various aspects of quantum transport with respect to potential spintronic applications in two different kind of 2-dimensional electron gases (2DEGs) in GaAs/AlGaAs crystals.
First, we try to combine the advantages of optically active InAs quantum dots (QDs) and the versatile possibilities of gate tunable 2DEGs in a novel hybrid heterostructure. We have characterized a series of hybrid wafers with different spacings between the inverted 2DEG and Stranski-Krastanov grown self-assembled InAs QDs. Depending on this distance and actual QD nucleation – verified with atomic force microscopy on the wafer surface – the 2DEG mobility is reduced due to scattering induced by the QDs and the InAs wetting layer. For a tunnel barrier of 45 nm, the 2DEG exhibits mobilities exceeding 500′000 cm2/(Vs) despite the presence of InAs QDs, while coherent tunneling between 2DEG and QDs is still permitted. Using a top down approach, lateral quantum point contact gates are aligned precisely to a single, specifically chosen InAs QD. The 1D conductance is not sensitive to QD charging events, but is dominated by a disorder potential, even suppressing quantization.
In the second part of the thesis, we study the control of spin-orbit (SO) interaction. This relativistic coupling of the electron spin to its momentum can be used for coherent spin manipulation, but at the same time also causes spin relaxation. Theory predicts a special symmetry, protecting spin from relaxation in diffusive transport, when the two main contributions of SO interaction in GaAs quantum wells (QW) – the Rashba and the Dresselhaus effect – are of equal strength. We demonstrate broad, independent control of all relevant SO fields, allowing us to tune into this regime. By electrically locking the Rashba and Dresselhaus SO fields via top and back gate, we achieve spin protection for a wide range of voltages on a single QW. We use quantum corrections to 2D conductivity as a sensitive probe of SO coupling. The combination of transport data and numerical calculations allows us to quantify the relevant SO coefficients.